With the rapid development of the electronics, communications, and informational industries, market demands for power sources of electronics products have increased. Lithium ion rechargeable batteries with its many excellent properties are being broadly used to fill this demand. With their widespread use, consumers want higher battery performance, especially excellent overall battery properties. That is, the market is demanding that the lithium ion rechargeable battery have high discharge capacity, long cycle life, and excellent high current and safety characteristics.
For the lithium ion rechargeable battery, the active material for the negative electrode is critical in the battery's ability to increase the discharge capacity and cycle life. The stability of the underlying structure of the active material for the negative electrode is also important in assuring that the battery possesses excellent high current and safety characteristics.
At present, graphite materials are most commonly used as the active materials for the negative electrode of the lithium ion rechargeable battery. However, most graphite needs to be appropriately treated and modified before its can be widely used. Among the most common treatment methods, coating produces the best results. Many researchers have attempted to coat the surface of the graphite with a layer of polymer material and the carbonizing it at a predetermined temperature to obtain a “core-shell” structure for the graphite composite. This retains the reversible specific capacity and better charging and discharging platform characteristics of the graphite, but also incorporates the good compatibility of the polymer thermally decomposed carbon with the organic electrolyte solution. From that, they achieve the excellent qualities of long cycle life, good high current characteristics, safety and stability. However, in practical applications, the improvement in the overall properties is not ideal. For example, Batteries, 2002.32(1): 13-15, describes the use of epoxy resin and thermally hardened and decomposed carbon to modify crystalline flake natural graphite in multiple modification steps. This changed the form of the graphite and reduces the directional properties of the crystalline flake graphite. It also improves the surface morphology of the graphite, and improves the compatibility of the graphite and the electrolyte solution such that there is improvement in its cycle characteristics. However, the modified graphite obtained by this method has lower initial charging and discharging efficiency (approximately 80%) and cannot satisfy the market demand for this desired battery characteristic. In Japanese Patent JP10-012241, a CVD (Chemical Vapor Deposition) process is used to treat graphite for use as negative electrodes in batteries. However, its cycle characteristics are poor and the equipment for the treatment is complex and expensive, resulting in high production cost and difficulty in implementing this technology for industrial production.
Therefore, many researchers have started to research using multiple modification steps to modify different materials for the active material of the negative electrode. For example, CN1186350 involves combining carbon material A and material B, a metal with high lithium content or oxidized tin material, to form the active material for the negative electrode of the battery. A battery made with this material as its negative electrode has high reversible specific capacity. However, its cycle characteristics are still poor; its capacity retention rate is less than 90% after 20 charge and discharge cycles.
At present, several common problems are encountered when trying to improve the active material for the negative electrode of the lithium ion rechargeable battery. In attempting to improve the cycle stability characteristics, the initial charging and discharging efficiency is sacrificed. The ability to increase the cycle life span is limited, resulting in a battery that does not meet the demands of practical applications. Alternatively, the technology and equipment for the improvements are so complex that they result in a high cost of production and it is difficult to implement the improvements for commercial production. In summary, poor overall performance is the problem encountered by the existing technology for the lithium ion rechargeable battery.